A light-emitting diode (LED) has advantages of power-saving, long lifetime and small size, so that using light emitting diodes as a backlight source is already the current trend of the technological development of liquid crystal display (LCD). However, the light emitting diodes still have many application problems needed to be overcome.
Means for applying light emitting diodes to backlight module are generally sorted into edge-type and direct-type. With reference to FIG. 1, FIG. 1 discloses a conventional arrangement of light emitting diodes for a direct-type backlight module, wherein a plurality of light strings 90 are arranged side by side on a back of a liquid crystal panel as a light source of a backlight module, wherein the light strings 90 include a first light string 901, a second light string 902 and a third light string 903. Each of the light strings 90 is formed with a plurality of light emitting diodes 900 connected in series. One end of the light strings 90 are connected to a DC/DC converter 91 and the other end thereof are connected to a constant current control circuit. The DC/DC converter 91 converts a DC power source into proper voltage level for the light string 90 to use. The constant current control circuit 92 controls the operating current of each of the light strings through a voltage feedback to keep the operating current stable to prevent the light emitting diodes 900 from flickering.
However, the constant current control circuit 92 usually has different power loss on each of the light strings 90. This is because cost of classifying the light emitting diodes 900 according to device characteristic is too high, testing and classification on the forward voltage of each of the light emitting diodes 900 are generally not performed when arranging the light emitting diodes 900. With reference to FIG. 1, it shows that forward voltages VF of the light emitting diodes 900 of the first light string 901 are ranged from 3.1 volts to 3.5 volts; forward voltages VF of the light emitting diodes 900 of the second light string 902 are ranged from 3.1 volts to 3.4 volts; forward voltage VF of the light emitting diodes 900 of the third light string 903 are all 3.1 volts. Because one of the light emitting diodes 900 has a highest forward voltage VF of 3.5 volts, the DC/DC converter 91 must use this voltage value as a standard to drive all of the light emitting diodes 900, so that other light emitting diodes 900 having forward voltage less that 3.5 volts would have redundant power loss. Take the third light string 903 matching the least requirements as an example, the forward voltages VF of the light emitting diodes 900 thereof are all 3.1 volts, the lowest in forward voltage, wherein if the third light string 903 has five of said light emitting diodes 900 and the constant current is 120 mA, the third light string will have power loss reaching up to 5×(3.5−3.1)×0.12=0.24 W (Watts).
To overcome this problem, although each of the light string 90 is available to independently use one said DC/DC converter 91, so as to be provided different driving voltages according to forward-voltage characteristic of each said light string 90, such method will highly increase the cost of using the DC/DC converters 91 and does not match practical considerations in production cost.
Hence, it is necessary to provide a backlight module to overcome the problems existing in the conventional technology.